Add the ability to use Phrase App

https://code.google.com/p/otwarchive/issues/detail?id=4015

Phrase App is a translation application that we want to use on the i18n site to allow the translation group to generate translations for the archive.

There should be no change on the test site and beta.  However check the footers still work :)

Elz has commented upon this in the past

This also fixes an issue where the headers go though the translation system twice.
Is there a reason not to do this with a setting in ArchiveConfig? We don't configure anything else via environment variables AFAICR?